Reversible Oxidative Addition at Carbon.

The reactivity of N-heterocyclic carbenes (NHCs) and cyclic alkyl amino carbenes (cAACs) with arylboronate esters is reported. The reaction with NHCs leads to the reversible formation of thermally stable Lewis acid/base adducts Ar-B(OR)2 ⋅NHC (Add1-Add6). Addition of cAACMe to the catecholboronate esters 4-R-C6 H4 -Bcat (R=Me, OMe) also afforded the adducts 4-R-C6 H4 Bcat⋅cAACMe (Add7, R=Me and Add8, R=OMe), which react further at room temperature to give the cAACMe ring-expanded products RER1 and RER2. The boronate esters Ar-B(OR)2 of pinacol, neopentylglycol, and ethyleneglycol react with cAAC at RT via reversible B-C oxidative addition to the carbene carbon atom to afford cAACMe (B{OR}2 )(Ar) (BCA1-BCA6). NMR studies of cAACMe (Bneop)(4-Me-C6 H4 ) (BCA4) demonstrate the reversible nature of this oxidative addition process.